An air turbine starter is a device used to start a gas turbine engine, such as an aircraft jet engine. The air turbine starter is mounted by the jet engine, much as a starter for an automobile is located by the automobile's engine. In order to start the jet engine, the air turbine starter is activated. The owner of the present invention, Honeywell International Inc., has for years successfully designed, developed and manufactured air turbine starters.
An air turbine starter may include a high speed gear assembly that converts air flow into the rotational energy needed to start a gas turbine engine. This high speed gear assembly includes gears supported by rolling elements that rotate around a stationary roller shaft at speeds of about 26,000 rpm or more. Although prior air turbine starter designs are very safe and reliable, these high rotational speeds cause substantial Hertzian stresses and frictional shear loads on certain areas of the stationary roller shaft, which may result in wearing and fatigue at those areas. For years, stationary roller shafts of air turbine starters have been made from steel materials. However, because such steel materials generally have a hardness of 62 Rockwell C or less, they are susceptible to surface spalling and fatigue. Fabricating the stationary roller shafts from harder materials does not necessarily provide satisfactory results, as harder materials may also be more brittle and, thus, more susceptible to cracking. In addition, stationary roller shafts formed from steel materials with relatively high inclusion contents make the shafts susceptible to sub-surface rolling contact fatigue.
Accordingly, there is a need for an air turbine starter with a stationary roller shaft formed of material that is capable of substantially withstanding wearing and fatigue from rolling elements that rotate at high rotational velocities. The present invention satisfies this need and provides related advantages.